The secret is a one-time use to apply a set of policies to the pubkey the user is using.
So a user might choose to use their key A with nostrudel through nsecBunker, then nostrudel generates a key B..
Key B needs to be authorized to do things by Key A.
NsecBunker can just ask the user for permissions, or the user can create a “policy”, assign a single-use secret and provide that secret to nostrudel.
Just send it as the second (iirc) parameter on the connect call.
More info on the unmerged nip-46 PR on the nips repo about this token